The invention concerns the use of plasticized sulfur to prepare a flexible, fire-resistant roof shingle. In the past, the relative economics of asphalt and sulfur would not have suggested the usefulness of sulfur technology in the roof shingle industry. However, the recent availability of sulfur and the increased cost of petroleum-derived products have led to a variety of reasons to consider the application of sulfur.
Despite the economic incentives, the use of sulfur in roof shingles has been slow in developing due to a number of serious shortcomings. In particular, sulfur is an extremely brittle solid and will not withstand the stresses to which a roof shingle is subjected. For example, a roof shingle is nailed into position. Shingles made from sulfur must be pre-drilled to avoid splitting when nailed. In many applications, roof shingles need to be flexible enough to withstand the weight of a heavy snowfall at sub-zero temperatures. Again, shingles made from sulfur have not been strong enough to withstand considerable weight.
Perhaps the most serious shortcoming concerns the combustibility of sulfur. Sulfur roofing materials burn with excessive melting of the sulfur, which in turn flows and drips, transporting the fire with it. Thus, the fire is spread to other areas of the structure. Moreover, burning sulfur generates large amounts of sulfur oxides, posing a severe pollution problem.
Some attempts have been made to reduce the combustibility of sulfur by incorporating various flame retardants into the sulfur. For example, U.S. Pat. No. 1,835,767, granted Dec. 8, 1931, describes the use of sulfur resins to retard the combustion of sulfur. However, when the composition is applied to a fibrous or paper backing the laminated product loses its flexibility, becoming very stiff. Similarly, Dale and Ludwig, "Fire-Retarding Elemental Sulfur", Journal of Materials, Vol. 2, No. 1, March 1967, p. 131, describe the effect of a variety of materials on the combustibility of sulfur with either styrene or dipentene dimercaptan. The "fire-proof" compositions were suggested as wall coatings, presumably because they form a firm, inflexible coating.
Attempts have been made to prepare flexible cloth-like products using plasticized sulfur as a third coating over the product. U.S. Pat. No. 3,619,258, granted Nov. 9, 1971, to Bennett, and U.S. Pat. No. 3,721,578, granted Mar. 20, 1973, to Bennett describe the use of thin coatings of plasticized-sulfur compositions over an asphalt-impregnated fabric to prepare a flexible, water-proof product. The thin coating of plasticized sulfur is used to improve the weather-proof characteristics of the final product.